Diabetic ketoacidosis: pathophysiology

People still die from diabetic ketoacidosis. Poor patient education is probably the most
important determinant of the incidence of the catastrophe that constitutes "DKA".
In several series, only about a fifth of patients with DKA are first-time presenters
with recently acquired Type I diabetes mellitus. The remainder are recognised diabetics
who are either noncompliant with insulin therapy, or have serious underlying illess that
precipitates DKA.

Most such patients have type I ("insulin dependent", "juvenile onset") diabetes mellitus,
but it has recently been increasingly recognised that patients with type II diabetes mellitus
may present with ketoacidosis, and that some such patients present with
"typical hyperosmolar nonketotic coma", but on closer inspection have varying degrees of ketoacidosis.

DKA is best seen as a disorder that follows on an imbalance between insulin levels and
levels of counterregulatory hormones. Put simply:

"Diabetic ketoacidosis is due to a marked deficiency of insulin in the face of high levels of hormones that
oppose the effects of insulin, particularly glucagon. Even small amounts of insulin can turn off ketoacid formation".

Many hormones antagonise the effects of insulin. These include:

glucagon

cortisol

oestrogen

growth hormone

catecholamines

In addition, several cytokines such as IL1, IL6 and TNF alpha antagonise the effects of
insulin. [J Biol Chem 2001 Jul 13;276(28):25889-93]
It is thus not surprising that many causes of stress and/or the systemic inflammatory response syndrome,
appear to precipitate DKA in patients lacking insulin. Mechanisms by which these hormones and cytokines
antagonise insulin are complex, including inhibition of insulin release (catecholamines), antagonistic metabolic
effects (decreased glycogen production, inhibition of glycolysis), and promotion of peripheral resistance to
the effects of insulin.

Persons presenting with DKA are often seriously ill, not only because DKA itself is a metabolic catastrophe,
but also because significant underlying infection or other disorders may be present. Common precipitants of
DKA are:

Patients with DKA have marked fluid and electrolyte deficits. They commonly have a fluid
deficit of nearly 100ml/kg, and need several hundred millimoles of potassium ion (3-5+mmol/kg)
and sodium (2-10mmol/kg), as well as being deficient in phosphage (1+ mmol/kg), and magnesium. Replacement of these deficits
is made more difficult due to a variety of factors, including the pH derangement that goes with DKA.
Mainly in children, an added concern is the uncommon occurrence of cerebral oedema,
thought by some to be related to hypotonic fluid replacement.

There are several mechanisms for fluid depletion in DKA. These include osmotic diuresis
due to hyperglycaemia, the vomiting commonly associated with DKA, and, eventually,
inability to take in fluid due to a diminished level of consciousness. Electrolyte depletion
is in part related to the osmotic diuresis. Potassium loss is also due to the acidotic state,
and the fact that, despite total body potassium depletion, serum potassium levels are often
high, predisposing to renal losses.

Ketoacidosis is an extension of normal physiological mechanisms that compensate for
starvation. Normally, in the fasting state, the body changes from metabolism based on
carbohydrate, to fat oxidation. Free fatty acids are produced in adipocytes, and transported
to the liver bound to albumin. There they are broken down into acetate, and then turned
into ketoacids (acetoacetate and beta-hydroxybutyrate).
The ketoacids are then exported from the liver to peripheral tissues (notably brain and
muscle) where they can be oxidised.
Note that during ketosis, a relatively small amount of acetone is produced, this giving
ketotic patients their typical smell, often described as 'fruity'.

DKA represents a derangement of the above mechanism. Despite vast amounts of circulating
glucose, this carbohydrate cannot be used owing to lack of insulin. Ketogenic pathways are
maximally "turned on", supply of ketones exceeds peripheral utilisation, and ketosis results.
(There are a few other clinical states where similar keto-acidosis is seen. One is in alcoholics, who
may present with marked ketosis, and a variable degree of either hypo- or mild hyperglycaemia.
Another is in some pregnant women, particularly associated with hyperemesis gravidarum).

The physiological mechanism of ketoacidosis is interesting. The rate-limiting step in
the manufacture of ketones in the liver is the transfer of fatty acids (acyl groups) from
Coenzyme A to carnitine. Carnitine acyl transferase I is the relevant enzyme, often
referred to as CAT-I. To a certain degree, increased levels of carnitine will drive this
transfer, but the main factor that inhibits CAT-I is the level of malonyl CoA in the
liver. High levels of malonyl CoA effectively turn off the enzyme.

Malonyl CoA is manufactured by another enzyme called Acetyl CoA carboxylase. Acetyl CoA
carboxylase activity is in turn regulated by the amount of citric acid in the cell. The
more the Krebs' cycle is whirling around (and citrate is being produced), the greater the
activity of Acetyl CoA carboxylase, which in turn results in inhibition of ketoacid production.
Turn off the supply of substrate into Krebs' cycle, and ketoacids are formed.

You can work out that in the fasted state, glycolysis is diminished, the flow of substrate
into the citric acid cycle drops, and ketone manufacture is turned on. This is unfortunately
just what happens in diabetic ketoacidosis.

We now understand how, in the midst of plenty, the liver cell in DKA cries 'starvation'
and produces ketones!
Both absence of insulin and excess glucagon result in inhibition of glycolysis. Such inhibition
not only raises glucose levels, but stimulates ketone formation. Let's look in more detail
at how these hormones inhibit glycolysis.

The marked hyperglycaemia seen associated with diabetic ketoacidosis (and that encountered in nonketotic
hyperosmolar coma) is not as straightforward as was once thought! The combination of insulin lack and
high glucagon levels has a variety of effects on the liver including:

Inhibition of glycolysis by glucagon

Glucagon excess and low insulin levels both appear to have similar effects in inhibiting glycolysis.
Glucagon ultimately has a potent inhibitory effect on the formation of fructose 2,6 bisphosphate . This product is very important, because it's an extremely potent allosteric regulator of a major rate-limiting enzyme in the glycolysis pathway,
phosphofructokinase (often abbreviated to "PFK1").
The effect of glucagon is well characterised. When glucagon binds its cell-surface receptor,
through a fairly direct G protein-receptor coupled mechanism, protein Kinase A is stimulated.
Then the fun really starts, because protein kinase A phosphorylates an important regulatory
enzyme called phospofructokinase 2 (PFK2 ). This latter protein is a strange duplicitous
enzyme - when phosphorylated it wears one face, quite different from the unphosporylated
enzyme. When phosphorylated, PFK2 acts as a phosphatase, but when un phosphorylated, it's a
kinase. Phosphorylated PFK2 takes the vitally important Fructose 2,6 bisphosphate
and lops off a phosphate to turn it into fructose 6 phosphate. The kinase form of PFK2
does the opposite, and results in the creation of more fructose 2,6 bisphosphate.
As we hinted above, fructose 2,6 bisphosphate is a potent allosteric stimulator of the
enzyme PFK1.

The bottom line is that glucagon lowers fructose 2,6 bisphosphate levels
and inhibits glycolysis; if glycolysis is inhibited,
then flow of carbon atoms into the citric acid cycle slows, and ketogenesis is stimulated.

Effects of insulin

The insulin effect is far less well characterised, although we know that the effect is opposite
to that of glucagon. It used to be thought that the main effect of insulin was mediated by a complex
pathway involving a kinase called MAPK. We now know that this pathway is important in
the long-term effects of insulin on cellular proliferation, but not the acute
metabolic effects.
The key regulator in the metabolic effects of insulin appears to
be the enzyme phosphatidylinositol 3-kinase (PI3K).
This in turn causes activation of a variety of kinases (atypical protein kinases C,
and protein kinase B), which have profound metabolic effects, including inhibition of glycolysis
and stimulation of glycogen synthesis.
[J Clin Endocrinol Metab 2001 Mar;86(3):972-9;
Philos Trans R Soc Lond B Biol Sci 1999 Feb 28;354(1382):485-95;
Diabetes Metab 1998 Dec;24(6):477-89]
Insulin raises fructose 2,6 bisphosphate levels by a mechanism that seems to depend on activation
of PI3K [J Biol Chem 1996 Sep 13;271(37):22289-92].

Note that this is not the whole story, because glucagon and insulin also have opposing
effects on several other enzymes, including pyruvate kinase, and enzymes involved in
glycogen synthesis/breakdown.

Death rates in DKA vary widely between published series, with death rates generally in
the range of one to ten percent, although higher rates have been reported! Such variation is likely due
to different reasons for presentation, and patients presenting at various stages during
the evolution of DKA. Differences in management are also likely to affect outcome.
Patients who are more likely to die include:

As noted, DKA in children may be associated with cerebral oedema. Although uncommon (~1%),
this complication may be associated with a high mortality rate (about 25% or more), and
a high rate of neurological complications in survivors. The pathogenesis is far from clear.
It has been noted that those who develop cerebral oedema are more likely to have a low arterial
partial pressure of carbon dioxide on admission [Glaser et al]. Some studies [Krane] suggest that cerebral oedema may even be present
on admission. The clinical picture in such cases is often one of initial improvement in
level of consciousness, followed by gradual decline over several hours, culminating in
sudden collapse, resuscitation, and an adverse outcome.

It is often asserted that over-vigorous rehydration (especially with relatively hypotonic
fluids) is the prime cause of cerebral oedema in such patients, but there is little or no
evidence to support this attractive contention. Implicating relatively hypotonic fluids in
the pathogenesis of this cerebral oedema is attractive because we have long known that
in the face of extracellular hypertonicity, brain cells undergo complex metabolic changes.
"Idiogenic osmoles" are produced in the brain to limit brain cell shrinkage. There is
increased intracellular production of osmotically active substances such as myoinositol and taurine . It seems logical that rapidly administered hypotonic fluid will rush into
brain cells and result in cerebral oedema. However, in experimental animals, aggressive insulin
therapy is more likely to be associated with cerebral oedema than is aggressive fluid therapy!
Nevertheless, current texts now generally caution one against over-vigorous fluid resuscitation
in children with DKA, recommending that one replenish the fluid deficit over 36 hours or more.
In addition, the old-fashioned tendency to give massive amounts of insulin is now considered
unacceptable.

The initial acidosis seen with DKA is usually almost entirely attributed to elevated
levels of ketoacids, which are strong anions. An equivalent way of viewing the acidosis
is that it is associated with a lowered strong ion difference.
Note that rapid repletion with large volumes of "unphysiological" fluid such as normal saline can be expected to worsen
the degree of acidosis; conversely, there are several arguments against the use of more
'physiological' solutions such as lactated Ringer's. We explore this controversial area
in a companion web page.